EP1331444B1 - Method for regulating a gas burner - Google Patents

Description

The invention relates to a method for controlling a gas burner, especially in heating systems with blower.

In heating systems with blower according to the prior art, the amount of fuel gas is adapted to the amount of air by means of a gas fitting. For this purpose, the air mass flow is usually measured by means of the pressure drop across a diaphragm and controlled by this control pressure, the fuel gas amount. This method for mixing fuel gas and air has the disadvantage that due to changing fuel gas composition, the excess air can vary; This can lead to high pollutant emissions during operation and starting difficulties.

From the EP 770 824 B1 and DE 19539568C1 It is known that the fuel gas-air mixture can be regulated by measuring the ionization current, which is dependent on the excess air and has its maximum in the case of stoichiometric combustion, and the mixture can be varied as a function of the ionization current signal. The problem here is that a relatively small signal must be measured very accurately.

In a method for controlling the fuel gas-air mixture according to EP 833 106 there is a flame sensor near a burner plate. By increasing the Excess air is caused to lift the flame, whereby a rough measure of the excess air is given.

Also known is a method in which the oxygen content in the exhaust gas of a gas burner is measured and the fuel gas-air mixture is controlled such that there is a certain proportion of oxygen in the exhaust gas. Such a method is described for example in US 5,190,454 described. It should be noted that oxygen sensors, which measure excess oxygen over a period of years, do not belong to the state of the art and the measurement signal changes only slightly in the working range.

The invention has for its object to provide a robust method for controlling a gas combustion plant, in which the fuel gas-air ratio is controlled so that a safe ignition and low pollutant emissions are guaranteed. For this purpose, a signal to be measured clearly is to be used. In known methods, the deviations of the signal values to be measured are very small, which often leads to erroneous measurements and regulations, especially in the case of aged sensors.

The aim of the invention is to avoid these disadvantages and to provide a clear, robust control method for a gas burner.

According to the invention this is achieved in the method of the type mentioned by the characterizing features of the independent claim.

The proposed measures ensures that from time to time a calibration of the fuel gas-air-composite takes place, in which the mixing ratio is adjusted such that a clean and safe combustion is ensured.

Another advantage of the method according to the invention is the lower power consumption of the blower, since the pressure loss for the volume flow measurement is superfluous. This also reduces the fan noise and a smaller, usually cheaper fan can be used. It is no longer necessary to differentiate between natural gas H, natural gas L and LPG through different components; Here, a corresponding default for the control is sufficient to start the burner. According to the optional features, an advantageous threshold for the calibration process can be determined.

The features of claims 2 and 3 describe advantageous method steps for carrying out the method. Thus, the fuel gas-air mixture can be enriched by increasing the fuel gas quantity or reducing the amount of air. For leaning of the mixture, the amount of fuel gas can be reduced or the amount of air can be increased. It is conceivable that in the method both the fuel gas, and air quantity is changed.

According to the features of claim 4, there is the advantage that the calibration process can then - outside of the intended cycles - can be performed when an unusually high carbon monoxide or hydrocarbon concentration is present. So it is conceivable, for example, that shortly after calibration due to a liquid-air admixture, the fuel gas-air ratio changes significantly and a high carbon monoxide or hydrocarbon concentration is formed in the exhaust gas. According to the features of claim 5, setpoints are also present prior to the initial implementation of the installed gas burner, so that the burner can be operated with these values until the first calibration is carried out.

According to the features of claim 6, there is the advantage that the function of the exhaust gas sensor can be checked. At the specified time, only air should be present at the exhaust gas sensor. If, on the other hand, carbon monoxide or hydrocarbons are measured at appreciable altitude, this is an indication that there is a sensor error; instead of carrying out a calibration with a defective sensor, it would then be better to use the previous setpoint values.

According to the features of claim 7 there is the advantage that the function of the exhaust gas sensor can be checked even when switching off the burner. At the time of switching off, there may be a slight increase in the signal value for a short time. Shortly after switching off, only air should be present at the exhaust gas sensor. If, on the other hand, carbon monoxide is measured at appreciable altitude, this is an indication that there is a sensor error.

According to claim 8, it can be determined whether the sensor provides plausible values during operation or whether the sensor is likely to be defective. If there is a defect, the burner is operated with default values stored in the control system or values determined in a previous calibration procedure or causes a fault shutdown.

According to the features of claim 9 there is the advantage that redundant means of carbon monoxide or hydrocarbon and Ionisationsstrommessung can be determined whether the system is in order. If the system determines that an inconsistency is occurring, the calibration is aborted and the control uses the old setpoints.

According to the features of claim 10 there is the advantage that, given the contradictory information, a lockout occurs and thus a risk is excluded.

Due to the features of claim 11, further evidence for a defect arise. The ionization current has its maximum at near-stoichiometric combustion. If the ionization current falls when enriched, the combustion is substoichiometric; the threshold would have been reached in this case long ago.

According to the features of claim 12, in both latter cases, the burner is turned off to avoid improper combustion.

• Fig. 1 ein Heizungsanlage zur Durchführung des erfindungsgemäßes Verfahrens und
• Fig. 2 den Zusammenhang zwischen Luftüberschuß und Kohlenmonoxidemission.

The invention will be described below with reference to the Fig. 1 and 2 explained the drawings. Show it:

• Fig. 1 a heating system for carrying out the inventive method and
• Fig. 2 the relationship between excess air and carbon monoxide emission.

A heating system according to Fig. 1 has a burner 1 with a surrounding heat exchanger 10, to which an exhaust pipe 9, in which an exhaust gas sensor 6 is connected. The burner 1, a fan 2 is connected upstream. On the input side of the blower 2 is an air intake line 13, in which also a fuel gas line 12, which is separated by a gas valve 4 from the fuel gas supply 11, extends. The gas valve 4 has an actuator 5. The fan 2 has a drive motor 7 with speed detection 8. Actuator 5, drive motor 7, speed detection 8 and exhaust gas sensor 6 are connected to a controller 3, which has a memory module 31 and computing module 32. Also with the control is an ionization electrode 14, which is positioned just above the burner 1, connected.

In burner operation, the control unit 3, e.g. due to a room thermostat, not shown, in conjunction with a flow temperature detection, also not shown in the computing module 32, a target power of the burner 1 calculated. In the memory module 31, a desired signal for the fuel gas and air quantity is stored to the target power. With these desired signals, the blower 2 is driven with its drive motor 7 and its speed detection and the gas valve 4 with its actuator 5, whereby a fuel gas-air mixture flows into the blower 2 and from there to the burner 1. The mixture is burned on the outer surface of the burner 1, flows through the heat exchanger 10 and then flows through the exhaust pipe 9 into the open air.

Fig. 2 shows the relationship between carbon monoxide concentration and combustion air ratio λ. In order to achieve complete combustion, a combustion air ratio λ of 1.0 is theoretically necessary. $$\mathrm{\lambda }\mathrm{=}\frac{{\mathrm{m}}_{\mathrm{L}}}{{\mathrm{m}}_{\mathrm{L}\mathrm{.}\min }}$$

Here, m _{L is} the actual air flow and m _{L, min is} the stoichiometric air flow. The combustion of hydrocarbons into carbon dioxide always produces carbon monoxide as an intermediate. Due to the limited reaction time in the heat affected zone and insufficient mixing of fuel gas and air, in practice, however, a certain excess air is necessary to ensure complete burnout. Therefore, a CO value of well over 1000 ppm is usually reached at just over-stoichiometric combustion. Only with an excess of air of about 10%, the carbon monoxide emissions in the fully reacted exhaust gas fall significantly and reach in conventional burners values below 100 ppm. As the air ratio increases, however, the combustion temperature drops because of the proportion of inert gases; the combustion reaction becomes slows down and it comes to the termination of the reaction at the heat exchanger. Therefore, from an air surplus of about 80%, a significant increase in carbon monoxide emissions can be observed.

Since in stoichiometric combustion (theoretically) the entire fuel is burned and no excess air is present, in this case the combustion temperature is maximum. With excess air, the proportion of inert gases is increased, whereby the combustion temperature decreases. This has the consequence that the nitrogen oxide emissions are maximum in stoichiometric combustion and decrease as the excess air is increased. The efficiency of a heating system is maximum in stoichiometric combustion and decreases when increasing the excess air, since the inert gases absorb heat losses and the residence time of the exhaust gas in the heat exchanger is reduced due to the increased flow rate, which is not compensated by the improved heat transfer. However, it must be taken into account that soot formation can occur both with near-stoichiometric combustion and with very large excess air. this deteriorates the heat transfer at the heat exchanger.

The above facts have the consequence that gas burners are preferably operated with a defined excess of air. In the embodiment, therefore, it is assumed that a target air ratio of about 1.25. In Fig. 2 this corresponds to the point D, which lies in a desired range C.

When burning: $${\stackrel{\mathrm{•}}{\mathrm{V}}}_{\mathrm{air}}\mathrm{=}{\mathrm{I}}_{\min }\mathrm{*}\mathrm{\lambda }\mathrm{*}{\stackrel{\mathrm{•}}{\mathrm{V}}}_{\mathrm{fuel\; gas}}$$

f(P) ist hierbei die leistungsabhängige Verhältnisfunktion, die fast linear ist, zwischen Brenngas und Luft.Here, I _{min is} the minimum air requirement. Since the ratio of fuel gas to air over the entire modulation range does not have to be constant in a real burner system, a dependency results $${\stackrel{\mathrm{•}}{\mathrm{m}}}_{\mathrm{air}}\mathrm{=}\mathrm{f}\left(\mathrm{P}\right)\mathrm{*}{\stackrel{\mathrm{•}}{\mathrm{m}}}_{\mathrm{fuel\; gas}}$$

f (P) is the power-dependent ratio function, which is almost linear, between fuel gas and air.

bestimmt. Dieser Kalibriervorgang wird in festen Zyklen durchfahren.At the beginning of the calibration, there is any fuel gas / air ratio. The control 3 continuously controls the actuator 5 of the gas valve 4 in such a way that more and more fuel gas passes into the blower 2 at the same amount of air. As a result, the mixture is enriched; the air ratio drops. The exhaust gas sensor 6 measures the carbon monoxide emission in the exhaust pipe 9 and forwards the signal to the controller 3. If regulation 3 registers that the carbon monoxide emission has a threshold value of 2000 ppm given in the memory module 31 (point A in FIG Fig. 2 ), the mixture is not further enriched. It is known that such carbon monoxide emissions are achieved at an air ratio of about 1.08. Accordingly, the goal is to increase the air ratio by 0.17 to reach the target air ratio of 1.25. The control 3 is the speed of the fan 2 of the speed sensor 8 of the drive motor 7 and the position of the gas valve 4 (for example, via the timing of the actuator 5 in the form of pulse width modulation) known. These data are stored in the memory module. By comparing these data with reference values likewise stored in the memory module 31 in the calculation module 32, a correction factor k is set. It follows that the control 3 in the following demand-dependent operation, the fuel gas-air ratio according to the relationship $${\stackrel{\mathrm{•}}{\mathrm{m}}}_{\mathrm{air}}\mathrm{=}\mathrm{f}\left(\mathrm{P}\right)\mathrm{*}{\stackrel{\mathrm{•}}{\mathrm{m}}}_{\mathrm{fuel\; gas}}\left(\mathrm{P}\right)\mathrm{*}\mathrm{k}$$

certainly. This calibration process is run through in fixed cycles.

Due to the relatively large nominal range (C in Fig. 2 ) the measurement and control does not have to satisfy a particular accuracy. Thus, it is not a problem if, for example, instead of 2000 ppm 4000 ppm are measured, since the difference in the excess air for both carbon monoxide emissions are minimal. The emaciation of the mixture can be done in a relatively large tolerance band. It is known that commercially available burners, which are to be operated with lambda 1.25, can be operated without problems in a range between 1.20 and 1.30. It is again very easy to reduce the mixture in such a way that it controls this area with sufficient safety.

It may happen that, for example, by LPG-air admixture in winter, the fuel gas-air ratio changes within minutes. An unclean combustion would not be corrected until the next routine calibration, possibly even completely ignored, because at the time of the next calibration the original mixture would be available again. To prevent this, the exhaust gas sensor 6 also measures the carbon monoxide emission in the exhaust pipe 9 at certain intervals outside of the routine calibration. If a certain limit value (point B in FIG Fig. 2 ) is exceeded, then a calibration of the control 3 is initiated.

Optionally, in the calibration to change the mixture in the direction of fuel-rich composition instead of increasing the fuel gas amount and the amount of air can be reduced, while the amount of gas remains constant. Also, instead of an absolute carbon monoxide signal, a gradient (eg CO change per speed change of the blower) can be measured. The threshold value does not have to correspond to a specific CO-equivalent signal, but can also be determined, for example, according to background noise without CO (eg 20 mV) plus cut-off value (eg 0.5 V). In that case you would Assuming that the measured signal at carbon monoxide concentrations in the desired operating range are well below this threshold and the threshold value is an indication that a certain fuel gas to air ratio was in the direction of fuel-rich mixture below.

A further variant of the method according to the invention is that the calibration does not take place by an enrichment of the mixture up to a threshold value and subsequent leaning, but rather by a leaning of the mixture up to a threshold value and subsequent enrichment. This takes into account that - as from Fig. 2 visible - even with very low-fuel mixtures increase the carbon monoxide emissions. While the increase in carbon monoxide in fuel-rich mixtures is the beginning of the steep rise in almost all burners in the same λ range, the steep increase in fuel-lean mixtures is very burner-specific. This applies both to the beginning of the increase and to the gradient (ΔCO / Δλ).

It is also known that the emissions of unburned hydrocarbons behave in the same way in the exhaust gas as the carbon monoxide emissions. Therefore, in the method according to the invention also a sensor can be used, which generates a signal equivalent to the unburned hydrocarbons.

Claims (12)

1. Method for controlling a gas burner (1), in particular with a fan (2), comprising an electronic controller (3), which, at a predefined burner output, defines a target signal for the amount of burnable gas and the amount of air, comprising a device for controlling the amount of burnable gas (4, 5), and comprising a waste gas sensor (6), which generates a signal equivalent to the carbon monoxide concentration or to the concentration of unburned hydrocarbons, characterised in that, after a specific operating time or at periodic intervals, preferably burner start-up or absolute time, a calibration procedure is carried out, in which the burnable gas/air mixture is made richer or leaner until the signal of the waste gas sensor (6) or a gradient of this signal, preferably derived according to the fan speed or the control signal of the device for controlling the amount of burnable gas (4, 5), exceeds a predefined or calculated threshold value, whereby it is known that a specific air ratio is present, in which state the signal for the amount of burnable gas and the amount of air is detected and then the burnable gas/air mixture is again made leaner or richer at a predefined ratio, whereby new target values for the amount of burnable gas and amount of air are defined.
2. Method for controlling a gas burner (1) according to claim 1, characterised in that the burnable gas/air mixture is made richer and/or leaner by changing the control signal for the amount of burnable gas.
3. Method for controlling a gas burner (1) according to one of claims 1 or 2, characterised in that the burnable gas/air mixture is made richer and/or leaner by changing the control signal for the amount of air.
4. Method for controlling a gas burner (1) according to one of claims 1 to 3, characterised in that the calibration procedure is initiated if the signal equivalent to the carbon monoxide concentration or to the hydrocarbon concentration lies above a predefined threshold value during operation of the gas burner at least 15 seconds after burner start-up.
5. Method for controlling a gas burner (1) according to one of claims 1 to 4, characterised in that target values for burnable gas and air are predefined at the start of the control method (3).
6. Method for controlling a gas burner (1) according to one of claims 1 to 5, characterised in that the signal equivalent to the carbon monoxide concentration or to the concentration of unburned hydrocarbons is detected before burner start-up, preferably at least 5 seconds after fan start-up, and the calibration procedure is not initiated with the presence of a signal corresponding to a concentration preferably greater than 20 ppm.
7. Method for controlling a gas burner (1) according to one of claims 1 to 6, characterised in that, once the burnable gas feed has been disconnected, preferably after a short measurement break, preferably 1 second, the signal equivalent to the carbon monoxide concentration or to the concentration of unburned hydrocarbons is detected, and the calibration procedure is not initiated with the presence of a signal corresponding to a concentration preferably greater than 20 ppm.
8. Method for controlling a gas burner (1) according to one of claims 1 to 7, characterised in that, with the presence of a signal of the waste gas sensor (6) below a predefined threshold value, preferably 10 mV, or above another predefined threshold value, preferably 1 V, or with the presence of a gradient of this signal greater than a third threshold value, preferably 100 mV/sec, the calibration procedure is not initiated and the controller (3) controls the gas burner (1) using predefined standard values or values established in a prior calibration procedure, or the controller (3) disconnects the gas burner (1).
9. Method for controlling a gas burner (1) according to one of claims 1 to 8, characterised in that, in addition, the ionisation current is received at an ionisation electrode (14) located in the flame region, and, if the ionisation current increases in the case of a fuelrich burnable gas/air mixture whilst the signal equivalent to carbon monoxide or to hydrocarbon decreases, the calibration procedure is aborted.
10. Method for controlling a gas burner (1) according to claim 9, characterised in that the burner (1) is disconnected.
11. Method for controlling a gas burner (1) according to one of claims 1 to 10, characterised in that, in addition, the ionisation current is received at an ionisation electrode (14) located in the flame region, and, if the ionisation current decreases in the case of a fuelrich burnable gas/air mixture whilst the threshold value for the signal equivalent to carbon monoxide or to hydrocarbon is not reached, the calibration procedure is aborted.
12. Method for controlling a gas burner (1) according to claim 9 or 11, characterised in that the controller (3) controls the device for controlling the amount of burnable gas (4, 5), in such a way that the flow of burnable gas is interrupted

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